Ok, here's a different question: What is the relationship between boost added to an engine, via TC or SC or leaf blower () to the final compression of the engine at boost? Eg, if we add, say, 5 Lbs of boost to a 10.5:1 compression motor, what compression results? Is it a straight adder, like 10.5 + 5 = 15.5:1, or is it a ratio?

 

Second question, specifically for Rotarygod: You have repeatedly emphasised that a properly sized unit is critical for a particular motor. That being the case, what size TC specifically is the "right" one for the Renesis? T03? T04? Leafblower? :D IOW, if you were to build the ideal setup for the '8, what unit would it use, and why? Thanks...

 

All I can tell you is that it's not a direct adder. It's more complicated than that. Airflow considerations, compressor efficiencies, temperature, rpm, all contribute to the final compression ratio.

 

Ok, about what I figured. So how DO we determine what the final compression ratio is?

 

In a simple answer, no.

Static compression ratio of 10.5:1 will never change. You are referring to the effect of boost on the effective compression ratio. 14.7 psi is what the engine sees as pressure from the atmosphere on a "perfect" day of around 60 degrees at sea level. It's pretty close to this number. When forced induction is adding 10 lbs of boost, the engine actually sees 24.7 psi. we just assume that 14.7 psi is at 0 and start from there. Depending on the weather conditions this number may change either up or down even though the gauge still reads the same. There is a different amount of air on the engine. This is one reason why I stress that a published boost number in psi is really irrelevant when it comes to power. We don't have enough information.

I'm going to use a small number for example sake. By adding .5 psi to the above number of 14.7 psi, we are only theoretically adding around 3.4% more air to the engine. This would theoretically only raise the effective compression ratio by the same margin in percentage and bring it to 10.85:1. However this is all fine and dandy on paper by the actuality is that we probably have less than this due to temperature rise and timing. Since each engine is different, we really can't just use this simple formula to determine the effective compression ratio. We'll eventually hit a brick wall in effective compression ratio where we can't go any higher without higher octane gas. We will just get to a point where timing retard cancels out any benefits that any increase in boost will give us. Corky Bell gets into this type of math in his books so I won't deal with it here. It isn't just a set number that you can calculate but rather an educated guess based on theoretical situations. The biggest problem with automotive math is that just because it works on paper, doesn't mean that it works in real life.

Here is a link to a chart from BDS blower systems. I personally find the general trend of this chart to be correct but the functional truth of it to be far from realistic. It basically suggests that we can't safely run more than about 3 psi of boost on pump gas with a 10:1 compression ratio or that a 9:1 compression ratio engine can't run more than about 6 psi of boost. I have run 15 lbs of boost on my 9:1 compression RX-7 on pump gas with no issues what-so-ever and I can probably go higher. The problem with this chart is that nowhere does it mention the effects of compressor efficiency or the effects of timing changes. It is because of these additional factors that make it hard to determine what the effective ratio will actually end up being. Here is the chart anyways. Don't rely on it. Just use it to show the example of how less boost and more static compression in equal to lower compression and higher boost.

http://www.goodvibesracing.com/Compression%20Ratio.htm

 

A T3 based turbo is way too small for a rotary. If you are using 2 of them, you can make them work.

The properly sized unit will really be dependent on what application you are using it for. If the engine is being setup for drag racing where huge top end power is all that is neccessary then a larger turbo will be needed. If it is for low to midrange power with moderate boost then a much smaller turbo is needed. There is no one right turbo for the engine. It is a function of how much boost you want to run, where you want the power to be centered at, and how soon you want the turbo to spool up.

Depending on the situation I would use different turbos. For a good power increase for street use that is easily drivable but not really laggy, I would use a rather small T-04B or 60-1 HIFI for the compressor. These can flow enough air for about 400 hp but would probably be most efficient around the 350-375 range. Yes they can get more but you are really spinning them up and need a very efficient intercooler. On the exhaust side I would use a P trim turbine with a single inlet. I don't want the 2 seperate exhaust paths entering the turbo that are so common today. With 3 exhaust ports there is no point. I would start out in the vicinity of a .8X (insert numeral for X) or larger housing and adjust the size on a dyno to be sure it fits my needs. This is where I would start for street use. For higher power I would start looking at other options. I would also use a ball bearing turbo if at all possible with both oil and water cooling. You can get away with only oil but water lowers the risk of oil coking within the turbo. That setup would give a very nice drivable street car. Yes there are better turbos for more power but this would be responsive with a good deal of lowend and quick spoolup time.

I'm sure others will disagree on this selection but many people are hooked on the fact that larger must be better. Many of the RX-7 crowd have turbos that are far too large and are designed to be most efficient in a higher boost range than they are even running. My friend Brian in Dallas has a streetport 13B with 8.5:1 static compression ratio, a Haltech computer, air/water intercooler, and a 60-1 HIFI turbo. The HIFI is the SMALL 60-1. His best power number to date on that setup was 424.6 rwhp at 16 psi. That is pretty damn good. By contrast a T-66 or T-76 turbo typically can't hit this number at this low a boost level. It is all about efficiency. Run those turbos in the mid to upper 20's for boost though and hold on! Brian's car got this number from good tuning and the fact that the air/water intercooler has an ice tank. There was frost on the intercooler. This is why I say a realistic number from this setup on the average street car to be set in the 350-375 range but approaching 400. For a clear majority of the people this is more than enough and will easily propel a sub 3000 lb car to low 12's in the quarter mile. Why go any larger for street use? This same turbo isn't the best choice for a bridgeport 13B though. It will still get around the same peak horsepower but the midrange will be much nicer. While good for street use, the top end number is compressor flow limited.

 

Thanks for the great answers as always, RG!

Ok, regarding your boost versus compression answer, is there some way to estimate what the final compression ratio will be at boost?

And as for the ideal FI setup, that was exactly what I'm after. Now when the kits start to come out, I'll be looking first at what kind of turbine they used. Your "street" setup is exactly what I had in mind. :D

So what do you think of the ATI/ProCharger/Sunflower Mazda setup that's coming out?

 

While you were typing this I was probably updating my first response above. Scroll back up and see if you saw the whole thing.

 

Cool, that's what I was after. Thanks again!

 

rotarygod, how would your description of a street turbo kit change for a street/track combination use for the rx8?

 

Better top end power, less low end power. For track events, your average usable powerband is different than it is when you are just driving on the street. For track events you want more of your power in the top end where the rpms will stay most of the time. For street use it is important to not have power fall off on the top end but you may not necessarily need as much ultimate top end power since most of your driving isn't done here. This is why a smaller turbo is better in this application. For dual use you need to decide whihc is more important to you. Then you need to find a good middle ground. It may be as simple as just using a different exhaust housing on the turbo.

 

Is a 30 page Sticky really helping anyone? Seriously.

Aren't stickies supposed to be concise important topics that are moderated in order to maintain ease of use and topicality?

30 freakin pages!!!

 

No, stickies are topics that people like to talk about a lot, or topics that are of real interest or importance. 30 pages here is a prime example of the former.

 

It also keeps people from reposting the same topic over and over again and then people start yelling at people to search and then people start to cry and then....you get the point.

what ever happened to the electric super charager idea?? is that still floating around, or has it become vaporware already?

 

Great topic. I would like to see a solution to the low end torque problem with the 8.

 

Yes, this electronic supercharger is still out there and I have been talking to one of the forum members who is planning to try this out. I have talked with the manufactures of this product and they swear by it (of course they do) but the really interesting thing is when you read the testemonials on thier website about how well this thing works and then realize that a majority of the testemonials are from Porsche owners.

 

So, what will blow the motor first? 10 second blasts of e-scharging or nitrous? Whose doing the ECU maps? How long to market with a product that has predictable performance and reliability (not necessarily bullet-proofed) results? Will the e-charger reduce off-boost performance other than due to the weight penalty?

 

The one that will blow the motor first is the one that is tuned the worst. If both cars are tuned good, it doesn't matter. This of course assumes gains within reason.

 

The E-SC doesn't require any remapping of the ECU since it is only producing about 1.5-2lbs of boost at WOT. The unit doesn't even come on untill the accelorator is smashed to the floor. There's no chance of blowing the motor because it doesn't push enough boost.

YOu have a greater chance damaging the motor with nitrous for sure because your adding a highly combustable chemical in to the air/fuel mixture.

 

I will try to fond the mag its in. but Mike knight mad a electronic supercharger that was not so good from what I read. It coust as much as a superchatger? and you have to wait till it charges back up to boost?...

 

i love your signature quotes rotarygod. they're awesome.

 

I am new to this forum and am researching an RX8 as a replacement for my 96 Miata. I don't have the time or patience to sift through 30 pages, but it seeems no one has mentioned anyone actually doing FI on an 8 yet. I stopped by Sunflower Mazda in Olathe Kansas (Kansas City) a couple months ago and they had a car there they were putting a SC on that the sales guy claimed could make 400hp. It was a car they were building for a lawyer that was also getting a suspension makeover and had 19' Racing Hart wheels. He said they were one of only 2 dealerships in the nation authorized to do it right now (?) while still maintaining warranty. I didn't know much about them at the time, or that it was such a rare thing, or I would have investigated further. I can't even remember what company he said it was. I will be out of town for a few days, but will check back with them next week and see if I can dig up more info.

 

I got to stop by Sunflower Mazda today and get more info. They are using a centrifugal sc made by ProCharger, based down the street from them in Olathe, Kansas. For more info check out:
http://www.procharger.com/systems.shtml

They also had a silver 8 with the full MS body kit and 19" Racing Hart wheels, 245/35 front, 275/30 rear. Sweeet!

Oops, I just saw there's already another thread about Sunflower with pics.

 

Will a Greddy T78 fit in the RX-8's engine bay, provided some of the auxiliary junk in the engine bay is moved? Does anyone have the overall dimensions?

 

Problem is, you're in the wrong post to discuss specific FI kits that are in development. Search the entire Mazdaspeed/Aftermarket Performance forum, and you'll see this is a VERY common topic. Tons of threads out there.

Good info about the Sunflower kit too, but that also has a couple of threads discussing it in detail.